This invention relates generally to novel catalysts for use in the direct epoxidation of alkanes to form alkylene oxides, having particular utility in the conversion of propane to propylene oxide. The catalyst is prepared via precipitation and is a lanthanide-promoted, supported silver catalyst. The invention also relates to methods for catalyzing oxidative chlorination/halodehydrogenation and epoxidation reactions using the novel catalyst, and to methods for manufacturing the novel catalysts. The invention finds utility in the fields of catalysis.
Conversion of alkanes to oxygenates is typically considered to proceed via oxidative dehydrogenation of an alkane to an alkene followed by epoxidation of the alkene to provide an oxide in a separate process. A direct method of synthesizing alkylene oxides from an alkane such as propane has not been practical heretofore as activation of the propane requires high temperatures, which decompose partial oxidation products, particularly propylene oxide, or promote total oxidation.
Numerous studies have been conducted investigating the oxidative dehydrogenation of alkanes to produce alkenes. See, for example, Chaar et al. (1988) J. Catal. 109: 463, Siew Hew Sam et al. (1990) J. Catal. 123:417, and Stern et al. (1997) Appl. Catal. A: General, 153:21. Various magnesium vanadates are reported to yield propylene with particularly high selectivity and, when a second oxidic phase (Sb2O4) is added, selectivities of up to 95% have been achieved. Carrazan et al. (1997) ACS Symp. Ser. 638:223. The mechanisms by which the oxidative dehydrogenation occurs and the interactions responsible for high selectivity are yet to be identified.
Other than vanadium-based catalysts, several molybdenum and niobium catalysts have also been investigated for use in oxidative dehydrogenation. See, for example, Breitescheidel et al. (1991) Chem. Mater. 3:559, Geenen et al. (1982) J. Catal. 77:499 and Toreis et al. (1987) J. Catal. 108:161. Bettahar et al. (1996) Appl. Catal. A: General 145:1 and Kim et al. (1991) Appl. Catal. 70:175 discloses that molybdates of, for example, nickel or cobalt, yield acrolein in substantial amounts, thereby decreasing the selectivity toward propylene. Niobium oxide, particularly in combination with vanadium or molybdenum, has been shown to have high selectivity for propylene in the oxidative dehydrogenation of propane in Savary et al. (1997) J. Catal. 169:287.
Catalyst compositions without molybdenum or vanadium have also been known to perform oxidative dehydrogenation. Ji et al. (1996) Catal. Latt. 39:247 and Wang et al. (1995) J. Catal. 151:155 discuss the selectivity of combinations of lanthanum oxide, alkaline earth metal, and alkali metal. Unfortunately, temperatures in excess of 400xc2x0 C. are required for any of the above-mentioned catalysts to have significant activity and such temperatures result in decomposition of the propylene oxide.
Catalysts composed of lanthanum carbonate and chromium oxide have been shown to be active and selective at lower temperatures but have only been used in the oxidative dehydrogenation of isobutane, see Hoang et al. (1997) J. Catal. 171:313. Carbonate-supported catalysts are currently used in ethylene epoxidation and often contain reduced silver and an xcex1-alumina carrier. Catalysts of this nature have been described in U.S. Pat. No. 4,248,740 to Mitsuhata et al. and U.S. Pat. No. 4,342,667 to Armstrong et al.
It has now been unexpectedly discovered that a highly selective catalyst capable of xe2x80x9cone-potxe2x80x9d conversion of an alkane to an alkylene oxide can be obtained by using an alkaline earth metal carbonate as a support in combination with a rare earth metal promoter. Also surprising is the finding that such catalysts are capable of the selective oxidative dehydrogenation of alkanes at temperatures under 400xc2x0 C.
Accordingly, it is a primary object of the invention to provide a process for the conversion of an alkane to an alkylene oxide at temperatures less than 400xc2x0 C.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In a first embodiment, a novel process for the conversion of alkane to alkylene oxide is provided wherein an alkane and oxygen-containing gas feedstream contacts an alkaline earth metal carbonate-supported silver catalyst comprised of a catalytically effective amount of silver, and a promoting amount of a lanthanide metal promoter, an alkali metal halide, an alkali metal nitrate, and an optional transition metal promoter.
In another embodiment of the invention, a novel process for the conversion of propane to propylene oxide is provided wherein a propane and oxygen-containing gas feedstream contacts an alkaline earth metal carbonate-supported silver catalyst that has a catalytically effective amount of silver, and a promoting amount of a lanthanide metal promoter, an alkali metal halide, an alkali metal nitrate, and an optional transition metal promoter.
In a further embodiment of the invention, a novel catalyst composition is provided comprising an alkaline earth metal carbonate support, a catalytically effective amount of silver, an effective promoting amount of a lanthanide metal promoter, an effective promoting amount of an alkali metal halide, an effective promoting amount an alkali metal nitrate, and a promoting amount of a transition metal promoter.
In yet another embodiment of the invention, a novel process for the conversion of alkane to alkene is provided comprising contacting, at a temperature in the range of approximately 200xc2x0 C. to 400xc2x0 C., a feedstream, comprised of alkane and an oxygen-containing gas, and a supported silver catalyst, comprised of an inert refractory solid support comprised of alkaline earth metal carbonate, a catalytically effective amount of silver, an effective promoting amount of a halide anion, an effective promoting amount of a rare earth metal promoter, an effective promoting amount of a sodium promoter, and an optional effective promoting amount of a transition metal promoter.
In still another embodiment of the invention, a novel process for the conversion of propane to propylene is provided comprising contacting, at a temperature in the range of approximately 200xc2x0 C. to 400xc2x0 C., a feedstream, comprised of propane and an oxygen-containing gas, and a supported silver catalyst comprised of an inert refractory solid support comprised of alkaline earth metal carbonate, a catalytically effective amount of silver, an effective promoting amount of a halide anion, an effective promoting amount of a rare earth metal promoter, an effective promoting amount of a sodium promoter, and an optional effective promoting amount of a transition metal promoter.